Computer users are generally able to customize the graphical user interfaces of computer applications. Additionally, many online applications, such as web-based email applications allow users to customize their interfaces or appearance of the online application with different colors, fonts, images, line widths, and the like. For example, with certain online applications, users may select a theme from a menu of different themes to be applied to the appearance of an online application. Themes often include associated images for elements of the theme/application (such as icons to show that an email message is new) and define particular colors associated with those various images. Conventionally, the style and appearance of a theme (and its elements) are represented in one or more corresponding cascading style sheets (CSS). Typically, the CSS for a theme defines the theme's image shapes and their associated colors as hard-coded values. Images were edited manually (e.g., in Photoshop), one by one, for each theme and referenced directly from the CSS. Accordingly, in such systems it is time-consuming and inefficient to change the color of an image used in the CSS as this requires modification/reprogramming of the CSS code that defines the color of the image.